The present invention relates generally to chemical analysis (e.g. by gas chromatography), and in particular to a compact chemical preconcentrator formed on a substrate with a heatable sorptive zone that can be used to accumulate and concentrate one or more sorbents over time and then rapidly release the concentrated chemical species upon demand for chemical analysis.
Presently, there is a need for autonomous, portable, hand-held chemical analysis systems for the rapid and sensitive detection of particular chemicals including pollutants, high explosives and chemical warfare agents. Such miniaturized chemical analysis systems, which have been termed “chemical laboratories on a chip”, are currently being developed based on gas chromatography. The requirements for these chemical analysis systems are that they provide a high chemical selectivity to discriminate against potential background interferents which may be present at up to a thousand-fold or more higher concentration, that the chemical analysis be performed on a short time scale (e.g. in a minute or less) and that the chemical analysis be performed with high sensitivity (e.g. at concentrations down to the part-per-billion level). Low electrical power consumption is also needed for field use over a prolonged time period.
Current gas-phase microanalytical systems typically comprise a gas chromatography column to separate the chemical species, or analyte, in a gas mixture, and a detector to detect the separated species. Such microanalytical systems can also include a chemical preconcentrator. The chemical preconcentrator serves the important function of selectively collecting and concentrating the analyte(s) of interest out of a large gas sample volume on a sorptive material at the inlet of the microanalytical system. In particular, selective analyte preconcentration is an essential step for early-warning, trace chemical detection in real-world, high-consequence environments where a high background of potentially interfering compounds exists. The chemical preconcentrator can deliver an extremely sharp analyte plug to the downstream gas chromatograph by taking advantage of the rapid, efficient heating of the sorbed analyte with a low-heat capacity, low-loss microheater. The very narrow temporal plug improves baseline separations, and therefore the signal-to-noise ratio and detectability of the particular chemical species of interest. Further, with a rapid enough release, there is a greatly reduced need for mechanical means of sample introduction, such as valving. See R. P. Manginell et al., “Recent Advancements in the Gas-Phase MicroChemLab,” Proc. of SPIE 5591, 44 (2004).
Several previous microfabricated chemical preconcentrators have used a heated planar membrane suspended from a substrate as the microheater, wherein the sorptive material is disposed as a layer on a surface of the membrane to sorb the analytes from a gas stream. See U.S. Pat. No. 6,171,378 to Manginell et al., or full wafer thick slats that are in intimate contact with beads having adsorbent disposed thereon and wherein the beads are adjacent to slats that act as the microheater U.S. Pat. No. 6,914,220 to Tian et al. The slats disclosed by Tian et al. are the full thickness of the substrate in order to maximize the surface area of the beads that are coated within the sorbent zone, and therefore lose substantial heat to the substrate to which the slats attach.
Referring now to FIG. 1, prior art preconcentrator having a heated planar membrane suspended from a substrate wherein the sorptive material is disposed as a layer on a surface of the membrane to sorb the analytes from a gas stream is illustrated. A disadvantage of the design allows for heat loss via conductance from the heat elements through the membrane to the substrate along the entire perimeter of the sorbent. Reducing heat capacity and reducing heat loss between the preconcentrator and the substrate would allow for faster desorption temperature ramps at lower power.
Selective sample preconcentration is an essential step for early-warning, real-world, trace analyte detection. We have previously developed a microfabricated planar preconcentrator and three dimensional preconcentrators to address these issues for a wide array of analytes. The thermal efficiency and low heat capacity of these designs make them well suited as a platform support for adsorbent materials. Once analyte is collected on the sorbent zone, integrated thin-film resistive heaters allow for rapid thermal desorption of the sample into an analytical chain for separation and detection. Rapid heating on the order of a second or less is important for it allows the sample pulse to be delivered in a very narrow temporal plug to a gas chromatographic (GC) separation channel. This improves separations, and therefore the signal-to-noise ratio (S/N) and lower-limit of detectability.